Causes And Analysis of Bearing Steel Ball Defect

Time：2021.05.27  Source：Roller Bearings Suppliers

Bearing steel balls is an important part of the bearing structure. If there are defects in the steel ball, it will directly affect the performance of the bearing.

Reasons for the defect of bearing steel ball blank

（1）Defects in raw materials, such as material cracks;

（2）the heating temperature is high and the speed is fast, and the temperature difference between inside and outside the forgings is too large to produce cracking;

（3）the final forging temperature is too low, resulting in cracking during forging; Wet cracking occurs when the final forging temperature is too high, the cooling rate is too fast or the spray cooling is used.

Rough steel ball and raw material testing

Decarburization and carbon - poor microstructure appeared on the surface of the crack, on both sides of the crack and at the tail of the crack. On both sides of the crack, one side of the crack is very serious decarbonization, which is composed of white pure ferrite and a small amount of pearlitic type tissue. The other side of the carbon-poor tissue is composed of a small amount of white pure ferrite and more pearlite-type tissue. The crack morphology shows a folding type propagation. The observation of the microstructure in the non-crack area and the surface microstructure of the steel ball did not show any carbon reduction. Observation of the crack inside can be seen in the middle of the gray-black oxide scale. Obtain the tail of the crack, the tail is sharp, and eliminate the forging wet crack. The deepest distance between the crack and the surface of the steel ball is about 0.61mm, relatively speaking, the crack is shallow. In the subsequent processing process, the crack and the carbon layer are all ground away, but the steel ball with a deeper crack will bring serious quality problems to the finished ball, so it should be picked out. Need WSBC 22320 E, Click here to learn more.

When the original material cracks are formed by the steel ball forging, the cracks expand further to the depths. As the surface cracks of the raw material appear, the continuity of the steel matrix is destroyed, and the surface cracks will occur here in the steel ball forming.

From the microstructure analysis of the forging billet steel ball, no abnormality was found, and the crack of the billet steel ball was ruled out to be caused by improper heating temperature. There is no fine mesh carbide in the steel ball structure, but a fine lamellar pearlite structure. This is related to the reasonable control of the cooling rate of the steel ball billet, and has nothing to do with the forging crack, and can indirectly prove that the crack of the steel ball billet is not caused by too fast cooling.

The whole forging steel ball forming process, the bar only for induction heating, no other heat treatment, single from the induction heating of the blank has not generated any decarburization, obviously, the crack decarburization is caused by the left material crack decarburization.

The cross-section morphology of the crack shows that the crack further changes its path along with the internal propagation, and the tail of the crack is sharp. Obviously, it has nothing to do with forging wet crack, because the crack tail of wet crack is more convex.

The ball is heated by induction for only a short time, and the subsequent cooling mist cooling is unlikely to form a large decarburized layer near the crack source.

What else is the crack related to the forging steel ball?

At the same time, there is no sign of decarburization on any surface other than the defect. Dong Changxing et al. believed that if decarburization was found near the crack, it indicated that the original material had defects. During the heating process, the tissue near the crack changed and white ferrite was easy to form. The surface crack defect of round steel is mainly related to the smelting and rolling process.

When there are problems in casting, it is easy to cause the core crack and subcutaneous bubble. Unsuitable drawing speed is easy to form surface and center cracks. In the process of rolling, these defects will eventually extend to the surface and form surface cracks with the reduction of rolled piece sections. In addition, improper rolling process, but also easy to produce surface cracks, and can make smelting crack expansion. Click here to learn more about WSBC 22224-E1.

Surface crack defects of raw material are the main reason of the result in the crack of the steel ball blank, in order to improve the quality of wind power bearing forged ball, steel mills should be continuous improve the smelting and rolling process, to ensure that the bearing steel metal continuity, uniformity, uniformity of the chemical composition, etc., to improve the quality of the steel ball billet, ensure the quality of wind power bearing steel ball of the finished product.

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